226 COMPARATIVE ANATOMY chap. 



the muscular part of the foot, between the coils of the intestine. 

 They may lie behind the " liver," or else, passing between its lobes, 

 spread out at the sides of and below the kidney. 



The epithelium which lines the gonads is, morphologically, the 

 endothelium of the secondary body cavity. The reproductive cells 

 may either be produced from any part of the epithelium of the gonad, 

 or from definite areas of this epithelium (Cephalopoda), which areas 

 may then be called germinal epithelium or germinal layers. It may 

 then appear as if the germinal gland lay in or on a special sac, 

 whereas this sac is, in reality, the gonad itself, and the germinal 

 gland is only the much-developed germinal layer of the gonad. 



The ripe reproductive cells become detached from their place of 

 formation, and fall into the cavity of the gonad, i.e. into a part of the 

 secondary body cavity, from which they pass out in various ways. 



2. The gonads either have separate ducts {GhitonidM, Monotocardia, 

 Pulmonata, Opisthohrandiia, Cephalopoda, many Lamellibranchia) or they 

 utilise the nephridia as ducts. In the latter case the genital products 

 either pass direct into the kidney, and reach the exterior through 

 the nephridial aperture (all Diotocardia, the Scaphopoda, and many 

 Lamellibranchia), or they first pass into the pericardium, and then are 

 ejected through the nephridia (Solenogasires). Where the gonads 

 open into the kidneys, their apertures may lie in various parts of 

 these organs ; either in the proximal part, which communicates with 

 the pericardium by means of the renal funnel, and is usually widened 

 into the renal sac, or in the distal part (ureter) which opens externally, 

 or into a shallow urogenital cloaca. 



The gonads therefore open into : — 



a. The pericardium (Solenogasires). 



b. The proximal or pericardial part of the kidney. 



c. The distal part or ureter of the kidney. 



d. The urogenital cloaca. Or : — 



e. They open externally, quite apart from the kidney. 



Paired gonads have paired ducts (Solenogasires, Lamellibranchia). 

 Where there is a single unpaired gonad, there is either a single 

 efferent renal duct, or a single renal duct is made use of (Gastropoda, 

 Scaphopoda, Cephalopoda, etc.) ; the duct is then always asymmetrical 

 and usually lies on the right side. A paired duct, belonging to an 

 unpaired genital gland, is, however, found in the Chitojiidce and in 

 many Cephalopoda. 



When the genital glands have special efferent ducts, various 

 sections of the latter may be differentiated into accessory sacs and 

 glands, copulatory apparatus, etc., which, especially in the Pulmonata, 

 Opisthobranchia, and Cephalopoda, transform the ducts into a very 

 complicated apparatus. In males, this complication arises through 

 the development of copulatory organs, and of special glands which 

 form the capsules of the spermatophores, and of seminal vesicles, etc. ; 

 in females, through the development of albuminous glands, shell 



